Integrated circuit chips, which are also referred to as dies, are often assembled in electronic packages that include the die and a substrate. The substrate has a gold plated or unmetallized ceramic surface to which the die is mechanically attached using an organic polymer-based adhesive. The surface is the floor of a cavity which is defined by one or more tiers that surround the surface. Alternatively, the surface is an entire side of the substrate, i.e., there is no cavity. Surrounding the surface, either on the tier(s) or co-planar with the surface, are wire bond pads to which the die is electrically connected to the substrate by wires that run from the die to the wire bond pads. In hybrid applications, wire bonding results in interconnections that are chip-to-chip, chip-to-passive component or the like. Representative devices that use the electronic packages are computers, automobiles, calculators and other consumer and industrial products.
High performance reliability of the mechanical bonding of the die to the substrate requires that adhesive: has good adhesion to the die and the surface; shows no resin-bleed; has good high temperature stability; and maintains adhesive bond integrity during temperature and humidity stress exposure.
The adhesive is often a silver filled epoxy or cyanate ester. A common problem with conventional silver filled epoxies is that the epoxy resin separates, or "bleeds", out of the adhesive during die attachment and/or curing of the adhesive. Resin bleed out is related to surface porosity, surface wettability and adhesive type, Ireland, Int. J. Hybrid Microelectronics, Vol 5, No 1, pp 1-4 (February 1982). Bleed out can be reduced by implementing a bake of the metallized substrate at elevated temperatures and reduced pressure prior to bonding to change the surface properties The die must be attached within a few hours which is not always possible. The bake process does not work satisfactorily on non-metallized surfaces. Epoxy based adhesives are unable to withstand high temperatures, e.g., greater than 200-220.degree. C., and therefore are unsuitable for high temperature applications.
The cyanate ester adhesives also have the problem of resin bleed out on metallized and unmetallized ceramic surfaces. Silver filled cyanate ester-based adhesives have a problem of adhesion degradation during temperature and humidity stress exposure at 85.degree./85% humidity which limits their use to packages that are hermetically sealed to keep out moisture.
An adhesive exhibiting reduced bleed out and which does not exhibit the above shortcomings is highly desirable.